Data processing installations require a power supply of very high reliability in terms of waveshape and amplitude. However, line power now available from utility organizations has been observed to be deteriorating in quality to the extent that, in numerous instances, it has become unacceptable for direct application to computer systems. Vagaries in line power stem from many causes but are categorized principally as line noise and out of specification voltage. Line noise may develop from a variety of perturbations, for example spikes may develop due to short circuits along the distribution lines, radio frequency interference, lightning or power factor corrections manifested as oscillatory ringing transients. These transients generally are in a range of 200%-400% of the normal voltage envelope. Under-voltage or over-voltage phenomena generally occur in conjunction with regulator activity and load changing on the power line.
With respect to the effects of these aberrations on computer operations, line noise is characterized by data errors, unprogrammed jumps and software/data file alterations. Momentary under- and over-voltage generally results in automatic computer power down.
A wide variety of techniques for accommodating unreliable power supplies have been available in the marketplace, which generally may be categorized as involving two types of three phase technologies, to wit, systems which recreate a waveform such as motor generators or uninterruptable power supplies (UPS) using a battery charger, batteries, inverter and static switch, and those systems which modify waveforms such as voltage regulators, spike suppressors and the like. The latter systems are basically ineffective in the treatment of all line conditions which may be encountered. Regulators, for example, incorporate feedback loops, the performance of which is too slow to render the devices effective in computer applications. With respect to the former, UPS systems are effective but of such complexity and attendant cost as to render them cost ineffective. Motor generators, simply, are too expensive.
For computer related performance, it is also important to provide an isolation of the power input to the regulator system so as to avoid catastrophic shut-down. Such isolation, of course, aids in the prevention of the passing through of common mode noise. Difficulties have been seen to arise where the regulators have been operated in shunt as opposed to series association with load inputs.
For many years, investigators have found interest in and have utilized constant voltage transformers as a regulating device. In their elementary form, these ferroresonant regulators comprise a non-linear saturable transformer in parallel with a capacitor which is supplied from a source through a linear reactor. The saturable transformer and the capacitor form a ferroresonant circuit wherein the inductive components operate beyond the knee of a conventional magnetization curve. These devices have been seen to hold considerable promise, inasmuch as they are inherently simple, requiring very few components. For example, it is the inherent nature of the ferroresonant transformer to handle all regulating, harmonic neutralizing and current-limiting functions thus permitting the noted simplification. Further, since all regulating and current limiting functions take place inside the ferroresonant transformer, the approach eliminates the need for feedback loops which, as noted above, are found in line voltage regulators. An absence of such loops provides for very reliable and stable current limit and regulation that are inherent to the device and not subject to change or alteration due to component failures. This lack of closed-loop feedback circuits makes the ferroresonant device quite insensitive to non-linear or pulse loads. Because the waveshape is completely recreated, transients and high speed phenomena cannot penetrate the ferroresonant devices.
A wide variety of literature has been generated concerning this approach to regulation, as is evidenced by the following papers:
I. Practical Equivalant Circuits for Electromagnetic Devices by Biega, The Electronic Engineer, June, 1967. PA1 II. Static-Magnetic Regulators--A Cure for Power Line "Spikes" by Kimball, Electronic Products, reprinted by Thomas and Skinner, Inc., Bulletin No. L-552. PA1 III. A New Feedback--Controlled Ferroresonant Regulator Employing a Unique Magnetic Component, Hart, IEEE Transactions on Magnetics, Vol. MAG-7 No. 3, September, 1971, pp 571-574. PA1 IV. A Feedback--Controlled Ferroresonant Voltage Regulator, Kakalec, IEEE Transactions on Magnetics, Vol. Mag-6, No. 1, March, 1970. PA1 V. Design Techniques for Ferroresonant Transformers by Workman, Jr. reprinted by Thomas and Skinner, Inc., Bulletin No. L-551. PA1 VI. Comparison of Inverter Circuits for Use in Fixed Frequency Uninterruptable Power Supplies by Braton and Powell, Instrument Society of America, ISA-76, International Conference and Exhibit, October 11-14, 1976.
While ferroresonant devices have found use in inverter applications and the like, their use as a line voltage regulator, per se, in conjunction with computer and other installations has not found favor. This principally is due to their statistically unreliable performance on unbalanced three phase loads; their tendency toward instability under certain three phase loading conditions; and their inability to provide high currents sometimes required in starting loads. Earlier designs also have tended to be unstable at light loads due to low input choke impedance. Three phase ferroresonant regulators have been observed to exhibit instability in developing a proper sinewave output. When instability results in a loss of a proper sinewave output for computer utilization, the computers necessarily are shut down. Where practical correction can be made available to overcome this deficiency, however, the devices hold promise of finding widespread use as a power conditioning system. If the single phase ferroresonant regulators could be made operable under all three phase loading conditions, if three phase ferroresonant regulators could be designed such that their outputs do not fall into non-sinewave patterns, the ferroresonant type regulators would exhibit a highly desirable voltage regulation technique. Unfortunately, however, the design of the ferroresonant regulators to avoid instability is one which is heueristic in nature and achieving a satisfactory result is an evasive endeavor.
A more recent aspect of the power requirements of computer facilities is concerned with the accommodation of high start-up surge currents in computer components. It is desirable that line power be regulated to provide a proper sinewave input to the computer facility as well as to provide over-voltage and under-voltage regulation. However, for the transient period of start-up, there is required a capability for delivering as much surge current as possible to the computer components. For example, a typical motor utilized in computer devices will draw three amps current under steady state condition while it may draw as much as 20 amps for a matter of seconds while it is starting up and developing proper speed. A traditional weakness of ferroresonant transformers has been that they are unable to supply such start-up surges. Further, inverter devices utilizing ferroresonant techniques are designed specifically not to pass high currents and to transfer to alternate power in the event of a call for surge currents.